1. Field of the Invention
The present invention relates to a non-oriented electrical steel sheet having a high permeability and excellent electrical properties that is usable as iron core material of electrical, equipment and to a method of producing the non-oriented electrical steel sheet.
2. Description of the Related Art
Against a backdrop of worldwide moves to protect the global environment by conserving electric power and other forms of energy, restricting use of Freon gas and other measures, there has arisen a rapidly spreading drive to enhance the efficiency of electrical equipment, particularly in the rotating machine, small transformer and other sectors that use non-oriented electrical steel sheet as a material for magnetic iron cores. This has also made non-oriented electrical steel sheet a target of calls for property improvement, i.e., demands for non-oriented electrical steel sheets with high magnetic permeability.
Conventionally, the effort to lower the core loss of non-oriented electrical steel sheet has generally relied on the method of increasing the content of Si or of Al or the like. This is based on the principle that increasing electrical resistance reduces eddy-current loss. This method has the disadvantage that it inevitably lowers permeability. To overcome this problem, the method of coarsening the crystal grain diameter of the hot-rolled sheet to enhance both permeability and core loss property is used.
On the other hand, in non-oriented electrical steel sheet that has a transformation, one conventional practice has been to terminate hot-rolling near the upper limit of the a region, thereby securing the required crystal grain diameter before cold rolling and, as a result, elevating the permeability of the product. It is from this standpoint that Japanese Unexamined Patent Publication No. 56(1981)-38420 teaches a method for increasing coarseness of the hot-rolled crystalline texture by finishing hot-rolling at a temperature at or below a temperature midway between the Ar3 point and the Ar1 point and carrying out coiling at a temperature of 680xc2x0 C. or higher. The improvement in the permeability of a non-oriented electrical steel sheet that can be achieved solely by controlling the hot-rolling conditions is, however, limited.
As methods for enhancing the permeability of non-oriented electrical steel sheet by improving the primary recrystallized texture, Japanese Unexamined Patent Publication No. 55(1980)-158252 teaches Sn addition, Japanese Unexamined Patent Publication No. 62(1987)-180014 teaches addition of Sn and Cu and Japanese Unexamined Patent Publication No. 59(1984)-100217 teaches addition of Sb as methods for enhancing crystalline texture so as to produce non-oriented electrical steel sheet excellent in electrical properties. The expense of adding a crystalline texture controlling element such as Sn, Cu or Sb is, however, by no means negligible. It has therefore been difficult to provide a non-oriented electrical steel sheet at low cost.
Measures implemented during the production process, such as innovations to the finish annealing cycle as taught by Japanese Unexamined Patent Publication No. 57(1982)-35626, have achieved core loss reduction but have not had much effect toward improving permeability.
The inventors conducted an in-depth analysis in search of means for overcoming the aforesaid limitations of the prior art. Through this study they discovered that a non-oriented electrical steel sheet excellent in both permeability and core loss can be produced using a low-alloy composition system, effecting controlled hot rolling, and controlling the electrical resistivity of the steel to within a definite range.
One object of the present invention is to overcome the problems of the prior art by providing a non-oriented electrical steel sheet excellent in permeability and a method of producing the non-oriented electrical steel sheet.
The gist of the present invention is as follows:
(1) A non-oriented electrical steel sheet excellent in permeability whose steel contains, in percentage by weight,
0.1%xe2x89xa6Sixe2x89xa61.0%
0.1%xe2x89xa6Mnxe2x89xa60.8%
0.4%xe2x89xa6Alxe2x89xa61.0%, and
the balance of Fe and unavoidable impurities,
and which has an ay transformation, an electrical resistivity of not less than 10xc3x9710xe2x88x928 xcexa9m and not greater than 32xc3x9710xe2x88x928 xcexa9m, and a permeability xcexc15/60 of not less than 1500 (Gauss/Oe).
(2) A hot-rolled sheet for a non-oriented electrical steel sheet whose steel contains, in percentage by weight,
0.1%xe2x89xa6Sixe2x89xa61.0%
0.1%xe2x89xa6Mnxe2x89xa60.8%
0.4%xe2x89xa6Alxe2x89xa61.0%, and
the balance of Fe and unavoidable impurities,
and which has an ay transformation, an electrical resistivity of not less than 10xc3x9710xe2x88x928 xcexa9m and not greater than 32xc3x9710xe2x88x928 xcexa9m, a recrystallized texture of a grain diameter viewed in a hot-rolled sheet cross-section of not less than 5 xcexcm and less than 50 xcexcm, and a worked texture of an area ratio viewed in a hot-rolled sheet cross-section of not greater than 80%.
(3) A hot-rolled sheet for a non-oriented electrical steel sheet whose steel contains, in percentage by weight,
0.1%xe2x89xa6Sixe2x89xa61.0%
0.1%xe2x89xa6Mnxe2x89xa60.8%
0.4%xe2x89xa6Alxe2x89xa61.0%, and
the balance of Fe and unavoidable impurities,
and which has an ay transformation, an electrical resistivity of not less than 10xc3x9710xe2x88x928 xcexa9m and not greater than 32xc3x9710xe2x88x928 xcexa9m, and a crystal grain diameter of not less than 50 xcexcm and not greater than 500 xcexcm.
(4) A method of producing a non-oriented electrical steel sheet excellent in permeability comprising the steps of subjecting a steel containing, in percentage by weight,
0.1%xe2x89xa6Sixe2x80x941.0%
0.1%xe2x89xa6Mnxe2x89xa60.8%
0.4%xe2x89xa6Alxe2x89xa61.0%, and
the balance of Fe and unavoidable impurities,
and having an at transformation and an electrical resistivity of not less than 10xc3x9710xe2x88x928 xcexa9m and not greater than 32xc3x9710xe2x88x928 xcexa9m to hot rolling at a hot-rolling finishing temperature in finish hot rolling of not less than 850xc2x0 C. and not greater than 1050xc2x0 C. and also not greater than (Ar3+Ar1)/2.
The present invention will now be explained in detail.
The inventors conducted an intensive study to overcome the drawbacks of the prior art and enable simultaneous achievement of low core loss and high flux density. As a result, they discovered that, in the case of a non-oriented electrical steel sheet having a transformation, when a steel containing 0.1%-1.0% of Si, 0.4%-1.0% of Al and 0.1%-0.8% of Mn is designed in composition to have an xcex1xcex transformation, the result is subjected to controlled hot rolling under specified conditions to form a distinctive hot-rolled texture, and the hot-rolled sheet is used as a starting material, a non-oriented electrical steel sheet excellent in both permeability and core loss property can be produced.
The conventional method used to improve the permeability of a non-oriented electrical steel sheet has been to coarsen the crystalline texture before cold rolling. In research leading to the present invention, the inventors confirmed that an additional improvement in permeability,can be obtained by conducting hot-rolled sheet annealing to coarsen the crystalline texture before cold rolling but they further discovered that non-oriented electrical steel sheet with permeability comparable to that of a material subjected to hot-rolled sheet annealing can be obtained by the present invention even if hot-rolled annealing is not conducted.
The steel composition will be explained first. Silicon (Si) is added to increase the specific resistance of the steel sheet and thereby lower eddy-current loss and improve core loss value. Si must be added to a content of not less than 0.1% because sufficient specific resistance cannot be obtained at less than 0.1%. The Si content is further limited to not greater than 1.0% because permeability decreases at a content in excess of 1%.
Manganese (Mn), like Si, also works to lower eddy-current loss by increasing the specific resistance of the steel sheet. A Mn content of not less than 0.1% is needed to obtain this effect. The Mn content is further limited to not greater than 0.8% because permeability decreases at a content in excess of 0.8%.
Aluminum (Al), like Si, also works to lower eddy-current loss by increasing the specific resistance of the steel sheet. Not less than 0.4% is preferably added particularly when low core loss is desired. An Al content of not less than 0.6% is preferable for elevating permeability and boosting electrical resistivity. The Al content is further limited to not greater than 1.0% because permeability decreases at a content in excess of 1.0%.
The electrical resistivity of the steel is defined as not less than 10xc3x9710xe2x88x928 xcexa9m because core loss property declines at an electrical resistivity of less than 10xc3x9710xe2x88x928 xcexa9m. The electrical resistivity is defined as not greater than 32xc3x9710xe2x88x928 xcexa9m because permeability declines at an electrical resistivity of greater than 32xc3x9710xe2x88x928 xcexa9m.
One or more of P, B, Ni, Sn, Cu and Sb can be incorporated in the steel in order to improve the mechanical properties, magnetic properties or anti-rust property of the product or for any of various other purposes. Addition of these elements does not detract from the effect of the present invention.
Carbon (C) content must be controlled to not more than 0.004%. When the C content exceeds 0.004%, the core loss property is degraded by magnetic aging during product use and, in addition, carbides produced by reaction with impurity elements inhibit crystal grain growth during finish annealing, and this also degrades core loss property. The C content, therefore, has to be limited to not more than 0.004%.
Sulfur (S) and nitrogen (N) partially reenter solid solution during slab heating in the hot rolling step. This results in formation of sulfides such as MnS and nitrides such as AlN during hot rolling. As these compounds inhibit grain growth during recrystallization annealing, the S and N contents are both preferably limited to not greater than 0.003%.
Phosphorous (P) improves the punching property of the product and is therefore added up to 0.1%. So long as Pxe2x89xa60.2%, no problem arises from the viewpoint of product magnetic properties.
The process conditions of the present invention will now be explained.
Since the steel of the present invention has ay transformation, its hot-rolling deformation resistance tends to fluctuate when the hot-rolling finishing temperature during hot rolling exceeds (Ar3+Ar1)/2. As this makes it impossible to obtain a hot-rolled steel sheet excellent in sheet thickness accuracy, the hot-rolling finishing temperature is therefore defined as not greater than (Ar3+Ar1)/2. When the hot-rolling finishing temperature exceeds 1050xc2x0 C., coiling at a temperature of 650xc2x0 C. or lower becomes difficult. The upper limit of the hot-rolling finishing temperature is therefore set at 1050xc2x0 C. and also set at (Ar3+Ar1)/2. When the hot-rolling finishing temperature is less than 850xc2x0 C., rolling becomes difficult owing to increased hot-rolling deformation resistance. The lower limit is therefore set at 850xc2x0 C.
A steel slab having the composition set out above is produced by converter refinement and continuous casting or ingotting-blooming. The steel slab is heated by a known method. The heated slab is hot rolled to a prescribed thickness.
The average grain diameter of the recrystallized texture viewed in a cross-section of the hot-rolled sheet must be not less than 5 xcexcm and not greater than 50 xcexcm and the area ratio of the worked texture viewed in a cross-section taken in the rolling direction of the hot-rolled sheet cross-section must be not greater than 80%. When the grain diameter of the hot-rolled sheet is less than 5 xcexcm, the high permeability aimed at by the present invention cannot be obtained. The grain diameter of the recrystallization grains of the hot-rolled sheet is therefore defined as not less than 5 xcexcm. When the grain diameter exceeds 50 xcexcm, a high permeability cannot be obtained in copresence with the worked texture. The upper limit is therefore set at 50 xcexcm.
In the present invention, the area ratio of the worked texture viewed in a cross-section taken in the rolling direction of the hot-rolled sheet cross-section must be not greater than 80%.
When this hot-rolled sheet is used, an excellent permeability xcexc{fraction (15/60)} of not less than 1500 Gauss/Oe can be obtained by a single cold rolling and annealing.
When the area ratio of the worked texture exceeds 80%, the surface condition of the product is degraded by occurrence of ridging after rolling. The area ratio is therefore defined as not greater than 80%. In the case of the hot-rolled sheet having the composition defined by the present invention, a high permeability is easier to obtain when some worked texture remains.
xe2x80x9cWorked texturexe2x80x9d, as used with respect to the present invention, refers to both portions of the texture having dislocations present at high density and exhibiting dark color by etching and elongated grains produced by rolling. xe2x80x9cRecrystallized texturexe2x80x9d, as used with respect to the present invention, means texture composed of isometric crystals.
The hot-rolled sheet can be subjected to hot-rolled sheet annealing to coarsen the crystalline texture before cold rolling. At this time, the grain diameter of the hot-rolled sheet must be not less than 50 xcexcm and not greater than 500 xcexcm. When the grain diameter of the hot-rolled crystalline texture after hot-rolled sheet annealing is less than 50 xcexcm, the hot-rolled sheet annealing produces no effect. A grain diameter of not less than 50 xcexcm is therefore required. When the grain diameter of the hot-rolled crystalline texture after hot-rolled sheet annealing is greater than 500 xcexcm, the surface condition of the steel sheet after cold rolling becomes inferior. The upper limit of the grain diameter is therefore defined as not greater than 500 xcexcm.
In order to prevent grain refinement by transformation, the hot-rolled sheet annealing is preferably conducted at a temperature not greater than the Ac1 point.
xe2x80x9cPermeability xcexc{fraction (15/60)}xe2x80x9d, as used with respect to the present invention, is the value obtained by measuring the permeability at an excitation flux density of 1.5 Tesla and a frequency of 60 Hz in Gauss as the flux density unit and Oe as the excitation magnetic field intensity and dividing the flux density by the excitation magnetic field intensity.